1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic type such as a copier, a printer, or a facsimile, in particular, relates to a wet-type image forming apparatus configured to form a toner image using liquid developer.
2. Description of the Related Art
In an image forming apparatus of electrophotographic type, an electrostatic latent image on a photoconductor is developed with toner using a developing device. The electrostatic latent image thus developed on the photoconductor is transferred onto a recording sheet or the like, thereby forming an image thereon. For such a process of transfer in the image forming apparatus, an electrostatic transfer method is generally employed.
When transferring the toner image onto a sheet, which is a target of the transfer, a voltage is applied using a transfer roller or the like from the underside of the sheet provided to face the photoconductor so as to form an electric field between the photoconductor and the recording sheet, thereby electrostatically attaching the toner image onto the recording sheet by means of this electric field.
Thereafter, the fixing device applies heat and pressure to the recording sheet, thereby fixing the transferred toner image onto the recording sheet.
Meanwhile, in recent years, a wet-type image forming apparatus has been known as an image forming apparatus required to achieve higher image quality and higher resolution such as an office printer for a massive amount of printing or an on-demand printing device. The wet-type image forming apparatus employs liquid developer which contains small-size toner particles and allows for less disturbance of a toner image. In the liquid developer thus employed, toner particles are dispersed in a liquid carrier of paraffin-base solvent or the like. In the steps of development and transfer, the toner particles are moved as a result of migration caused by an electric field in a toner layer constituted by the liquid carrier and the toner particles, thereby transferring the image onto the recording sheet.
In the wet-type image forming apparatus employing the liquid developer, the liquid carrier in the toner layer needs to be removed to attain secure fixing.
The liquid carrier in the toner layer is removed by volatilization thereof, removal thereof at the fixing member, penetration thereof into the sheet. However, it is known that when the toner layer with a small amount of the liquid carrier comes into contact with the fixing rollers, “offset”, which is a phenomenon in which toner particles are attached to the surface of a fixing roller, is likely to take place.
To address this, Japanese Laid-Open Patent Publication No. 11-194621, Japanese Laid-Open Patent Publication No. 11-272079, or Japanese Laid-Open Patent Publication No. 2000-003061 proposes a method of suppressing the offset by impregnating a recording sheet with a nonvolatile liquid carrier, which serves as a release agent.
In addition, Japanese Laid-Open Patent Publication No. 2007-163777 discloses a method of suppressing the offset by adjusting an amount of a nonvolatile liquid carrier to be removed using a roller coming into contact with the toner layer, before transferring onto the recording sheet.
However, in each of the above-described methods, the nonvolatile liquid carrier remains in the recording sheet. Even in the method of Japanese Laid-Open Patent Publication No. 2007-163777, the liquid carrier in the recording sheet is not completely removed. Hence, even though the offset can be suppressed, the liquid carrier thus remaining serves to inhibit the fixing. Thus, it is difficult to achieve both the suppressed offset and the secure fixing.